Longer wavelengths such as red are absorbed at a shallower depth than shorter wavelengths such as blue, which penetrates to a deeper depth. Because of this, the higher energy light with short wavelengths, such as blue, is able to penetrate more deeply.
Skin depth defines the distance a wave must travel before its amplitude has decayed by a factor of 1/e. The skin depth is the reciprocal of the decay constant β. Thus: δ=1β=1ω(μϵ2[(1+σ2ϵ2ω2)1/2−1])1/2. Since β depends on the frequency and the physical properties of the media, so does the skin depth.
A higher frequency will have a shorter wavelength. Penetration is directly related to wavelength. Smaller wavelengths are more easily reflected or refracted in the superficial tissues than longer wavelengths. As wavelength is increased (or frequency decreased) the ultrasound will penetrate deeper.
For example, the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm (1 to 1.5 inches). Radio waves have been applied to the body for 100 years in the medical therapy of diathermy for deep heating of body tissue, to promote increased blood flow and healing.
In superconductors, the London penetration depth (usually denoted as or ) characterizes the distance to which a magnetic field penetrates into a superconductor and becomes equal to −1. times that of the magnetic field at the surface of the superconductor. Typical values of λL range from 50 to 500 nm.
Such a minuscule amount of light penetrates beyond a depth of 200 meters that photosynthesis is no longer possible. The aphotic, or “midnight,” zone exists in depths below 1,000 meters (3,280 feet). Sunlight does not penetrate to these depths and the zone is bathed in darkness.
Higher energy means higher frequency. So lower frequency waves "penetrate" materials better, because photons with lower frequencies don't have enough energy and simply "pass through" without interacting with the matter.
Eddy current testing uses the principle of electromagnetic induction to detect flaws in conductive materials. An excitation coil carrying current is placed in proximity to the component to be inspected.
The eddy current method is based on the principle of generating circular electrical currents (eddy currents) in a conductive material. This is achieved by the use of a coil connected to an alternating current generator driving an alternating magnetic field (primary field).
The two major applications of eddy current testing are surface inspection and tubing inspections. Surface inspection is used extensively in the aerospace industry, but also in the petrochemical industry. The technique is very sensitive and can detect tight cracks.
The eddy current (its effect) in an electromagnet is very widely used for many applications, one of which is an ordinary electricity meter every house usually has.
Disadvantages of Eddy Currents:
There is a major heat loss during cycling eddy currents due to friction in the magnetic circuit, especially where the core is saturated. Thus there is the loss of useful electrical energy in the form of heat. There is magnetic flux leakage.Eddy current testing is a proven method for detecting flaws in metal structures. Besides being used to detect defects or flaws, ECT can also measure metal thickness. This is especially useful for metal tubing. Such testing can detect corrosion or any other damage that causes metal thinning.
Ideally, eddy current testing will occur approximately every 3 to 5 years, possibly more frequently if there was prior damage that should be monitored. Factoring eddy current testing into an annual inspection ensures that testing occurs with the proper regularity.
Eddy-current testing (also commonly seen as eddy current testing and ECT) is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials.
The advantages of eddy current testing are: Sensitivity to surface defects. Able to detect defects of 0.5mm in length under favourable conditions. Can detect through several layers.
Eddy currents are formed by a magnetic field acting on a electrically conductive material. You can increase the strength of the magnetic field to increase the effect of the eddy currents. Or you can use a more conductive material. Aluminum is often used; copper would be more effective.
Eddy current probes are most often shielded using magnetic shielding or eddy current shielding. Magnetically shielded probes have their coil surrounded by a ring of ferrite or other material with high permeability and low conductivity.
Component features (material thickness, geometry, etc.) should be the same in the reference standard as those in the test region of interest. If the reference standard is the type with intentional defects, these defects should be as representative of actual defects in the test component as possible.
Induction heating is the process of heating a material through the generation of eddy current. An external applied alternating magnetic field is applied, and heat is generated from the resistance to the eddy current (i.e., Joule heating).
Eddy currents are currents which circulate in conductors like swirling eddies in a stream. They are induced by changing magnetic fields and flow in closed loops, perpendicular to the plane of the magnetic field. Like any current flowing through a conductor, an eddy current will produce its own magnetic field.
Induced current would be the current that results in a conductor due to a moving magnetic field. Eddy current is when the induced electrical current then generate their own magnetic moments in that conducting core. These magnetic moments oppose the source magnetic field.
Eddy currents are named so because the current looks like eddies or whirlpools. When a conductor is placed in the changing magnetic field, the induced current in the conductor is termed as Eddy currents.
An eddy is a circular current of water.
If you've ever seen a small whirlpool of water when you paddle on a river, stream, bay, estuary, or ocean, this is an eddy. Eddys form wherever there are areas where current is impeded. They can form behind an obstruction like a boulder or structure like a dock or channel marker.In order to reduce the eddy current loss, the resistance of the core should be increased. In devices like transformers, the core is made up of thin sheets of steel, each lamination being insulated from others by a thin layer of varnish. As the laminations are thin, they will have relatively high resistance.
Eddy currents can have many uses. The heating effect of eddy current is used for melting a metal in an induction furnace. Eddy currents can also be harmful. It is well known that the rapidly changing magnetic fields can generate dangerous eddy currents in the human body.
The heating effect originates from the transformation of electric energy into heat energy and is used in induction heating devices, like some cookers and welders. The resistance felt by the eddy currents in a conductor causes Joule heating and the amount of heat generated is proportional to the current squared.
The heating effect of the eddy current is used for melting metal in an induction furnace. Eddy currents of large magnitude are produced field. The changes in the magnetic field are so rapid that very large eddy currents are generated and heat produced is sufficient to melt quickly.
Eddy current loss is conductive I2R loss produced by circulating currents induced in response to AC flux linkage, flowing against the internal resistance of the core.
